mips: io: remove duplicated codes

mips: io: remove duplicated codes

By adding <asm-generic/io.h> support, the duplicated phys_to_virt
can be removed to use the default version in <asm-gneneric/io.h>.

Meanwhile move isa_bus_to_virt() down below <asm-generic/io.h> including
to fix the compiling error of missing phys_to_virt definition.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Helge Deller <deller@gmx.de>
Cc: Serge Semin <fancer.lancer@gmail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>